Much human cancer originates in organs not vital to the life and well-being of the patient. Many of these malignant tumors retain to some extent the morphological and histological characteristics of the organs or tissues from which they are derived. We believe that such tumor cells are also likely to retain the antigenic characteristics of some of the normal cells of the tissues that underwent malignant change. If such an antigen is expressed on the surfaces of tumor cells, but not on cells vital to the life and well-being of the patient, this antigenic characteristic may provide a target for an attack on tumor by immunological mechanisms. We are looking for such antigens, in rats, by trying to prepare radioactive antibodies able, after i.v. injection, to localize with high specificity in tumors originating in mammary, large intestine, pancreas, and thyroid tissue. Antibodies will be sought in xenogeneic antisera. Radioactive tumor localizing antibody will be used as an immunological probe to identify the corresponding antigens during tumor cell fractionation studies. Such isolated antigens will then be used as immunogens to produce sera containing a larger fraction of tumor localizing antibody.